Lightweight refractory materials
As the main material for the lining and insulation layer of high-temperature kilns and high-temperature equipment, lightweight refractory materials have the characteristics of low volume density, low thermal conductivity and compressive strength, and high apparent porosity compared with dense refractory materials. They can effectively prevent heat flow from being transmitted outward through the enclosure structure and reduce the heat loss of high-temperature equipment such as kilns.
PART 01 Low volume density
The volume density refers to the ratio of the mass of the dry product to the total volume of the product. The volume density of lightweight refractory materials is very small, generally between 0.6 and 1.2 g/cm3; while the volume density of semi-lightweight materials is generally between 1.2 and 1.8 g/cm3. As one of the main factors for measuring lightweight materials, in the production of lightweight refractory materials, the volume density of the material is generally reduced as much as possible without meeting the use intensity of kilns or high-temperature equipment.
PART 02 Low thermal conductivity
Thermal conductivity is one of the main factors to measure the thermal insulation effect of refractory materials. Under the condition of ensuring the use strength of the material, the lowest possible thermal conductivity is conducive to maintaining the heat flow of the kiln or high-temperature equipment and reducing heat loss. Lightweight refractory materials have a large number of pores, and the gas in the pores has a good thermal insulation effect, which makes the heating effect obvious and the thermal conductivity is low; whether the pores inside the product are small and uniform directly determines the thermal conductivity and use strength of the material. Generally, the smaller the pore diameter, the better the thermal conductivity of the product. The thermal conductivity of lightweight refractory materials is relatively small, mostly less than 1.0W/(k·m)(600℃×3h), while the thermal conductivity of semi-lightweight refractory materials is generally less than 1.26W/(k·m)(600℃×3h).
PART 03 Low compressive strength
Lightweight refractory materials form an effective and dense connection. Therefore, compared with dense refractory materials, their compressive strength is low, and their wear resistance and erosion resistance are poor. In daily use, it is generally not used as a working layer but placed behind the working layer as an insulation layer. Therefore, on the basis of ensuring the use strength (for load-bearing), the bulk density and thermal conductivity will be reduced as much as possible.
PART 04 High apparent porosity
The most intuitive indicator of lightweight insulating refractory materials is pores. For lightweight insulating refractory materials, pores mainly come from external pore-forming agents, which are formed through foaming, burning or reaction. Generally, they can be divided into three types: (1) open pores; (2) closed pores; (3) through pores. Among them, through pores are the most harmful to the material, and closed pores have better insulation effect.
For lightweight insulating refractory materials, the apparent porosity refers to the ratio between the volume of all pores in the lightweight insulating refractory materials and their total volume. Generally speaking, the apparent porosity of lightweight insulating refractory materials is greater than 45%, and when preparing lightweight materials with a bulk density of about 0.5g/cm3, the apparent porosity is even as high as 80%.
PART 05 Widely used
-Generally used as the lining or insulation layer of industrial kilns: Lightweight refractory materials can reduce the heat loss of brick masonry of kilns and other equipment due to their low thermal conductivity, save energy, and effectively improve production efficiency. However, due to the large apparent porosity of the product, loose structure, low strength, poor resistance to various types of erosion and wear resistance, it can only be used for the lining, insulation layer and other parts of high-temperature thermal equipment such as kilns. It is generally not suitable for working linings and parts with large loads.
Insulation principle
Heat is a kind of energy, which is closely related to the movement of molecules, atoms, electrons, etc. of the constituent materials. The thermal phenomena of all substances are the result of the collision, transfer and movement of particles inside the substance. In thermal industries such as kilns, the energy loss due to heat transfer becomes extremely obvious. Generally, the temperature difference between the inside and outside of the kiln is more than 800℃. The role of lightweight insulating refractory materials is to slow down the heat transfer process through the internal pores of the gas, thereby achieving the effect of thermal insulation. Generally, lightweight insulating refractory materials are mainly composed of solid phase and gas phase, so the overall heat transfer process of the material consists of heat transfer between solid phases, heat transfer between solid phase and gas phase, convection between gas phases, radiation heat transfer between pores, etc. When the material is used at high temperature, radiation is the main factor affecting the thermal conductivity: when used at room temperature, radiation becomes a secondary factor.
The reason why lightweight insulating refractory materials have a smaller volume density, lower thermal conductivity and good thermal insulation effect is that their extremely high apparent porosity allows a large amount of air to exist in the pores of the material. The thermal conductivity of air is much lower than that of solids. When the air content in the material is high, the convective heat transfer between the gas phases in the pores will increase, and vice versa, the convective heat transfer will be reduced. Generally, there are many closed pores inside the structure of lightweight insulating refractory materials, so the convective heat transfer between the gas phases in the pores contributes less to the thermal conductivity of the material.
The higher the apparent porosity of lightweight insulating refractory materials, the higher the gas content in the pores inside the material, the lower the relative solid content, and the lower the effective thermal conductivity between the solid phase and the gas phase, and the better the thermal insulation performance of the material.
According to the Stephen-Boltzmann law (2) and Fourier's law (3), the contribution of radiation heat transfer to the effective thermal conductivity can be calculated.
When the material is used at high temperatures, the influence of radiation on the thermal conductivity is dominant. The effective thermal conductivity of the insulating material is the superposition of solid-gas phase heat transfer and radiation heat transfer.
The effective thermal conductivity of porous insulating materials is directly related to the porosity of the material, the thermal conductivity of the solid phase and the gas phase, and the thermodynamic average temperature of the material. The indirect effects of the insulating material during use can be well explained based on the above theory.
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